stitching uv islands creates sharp edges in new version
This appears to be because blender is keeping the old unwrap edge markings, but knows the UV is now different than what was created from the unwrap markings, so it is now highlighting them. This is not going to show up in the final render.
Edit: sorry, they are actually sharp marks and are not UV marks. But regardless, you can select those edges and then “clear sharp“ in… I don’t remember which menu. It’s either mesh or edge. (Why is this small?)